Feedback circuit arrangement



Y Patented Jan. 27,1942

O'swold 'Edward Keall, Summerseat,

West

Hanningiield, England, assignor to Radio Corporation of America, a corporation of Delaware Application January 31, 1940, Serial No. 316,525`

In Great Britain January 12, 1939 f 7 claims'. (ci. 179-111) This inventionrelates to electron discharge circuit arrangements and more particularly to circuit arrangements wherein voltage feed-back is employed.

'I'he invention is illustrated in and explained in connection with the accompanying diagrammatic drawing, wherein Figure 1 illustrates by a block diagram a conventional feed-back circuit a'rrangement; and

Fig. 2 illustrates certain improvements which constitute the substance lof my invention.

It is' often required to provide voltage fee'd- V back from one valve to a preceding valve inl a chain of valves, for example, to improve the frequency characteristic of an amplifier including said valves. To mention but one of the numer- ,ous cases wherein voltage feed-back is required, an oscillator unit may be arranged to feed into a multi-stage amplifier unit, and in order to improve the frequency characteristic of the amplifier, or to reduce non-rectilinear distortion, it is a common expedient to provide a voltage feed-back circuit across the amplifier from, say, the last valve thereof to the first valve. The method at present usually employed lfor thus introducing voltage feed-back is illustrated in block diagram form in Figure 1.

Referring to Fig. 1 rectangle O represents an oscillator having output terminals OTI and T2, and rectangle A represents an amplifierhaving discharge devices and a vpreceding point by means of one or more electron discharge devices presenting high mutual conductance and employed to vary the impedance of ja tube circuity of a preceding stage, particularly between the cathode and ground.

Preferably the cathode impedance common to` theV input and output circuits is an .ohmic resistance, though it may be a reactance. The cathode impedance may also becontrolled by an electron multiplier.- Known `electron multipliers' are available having mutual .conductances of the order of 40 milli-amperes per volt. Their use in carrying out this invention is, therefore, of sub- Ordinary tubes, though utilizable, do not as a rule have mutual conductances exceeding about 1/2 of this value. As will be hereinafter explained, the higher the mutual conductance of the feed-back circuit, or

cathode follower, the lower can be the value ofv the cathode resistance. With a cathode follower having a mutual conductance of the order of 40 milli-amperes per volt, a cathode resistance of as low as 10 ohms, may be employed. Again, as.

ywill be hereinafter explained, since this cathode resistance is common to the cathode circuit of the device at which voltage feed-back is effected,

it will produce a certain measureof current feedinput terminals ATI, AT2 and output terminals AT3, AT4. Voltage feed-back is introduced by interrupting the 'earth connection which, were no feed-back provided, would normally directly connect the low potential output terminal 0T2 of the oscillator unit 0 with the low potential input terminal AT2 of the amplier A, in order to Ainsert the feed-back line F'BL. This expedient, though very commonly employed, has 'the serious disadvantage that, by reason of the break in the earth connection for the insertion of the feed-:back line, only one of the two units-the oscillator and amplifier units-may be earthed. This defect is obviously a cause of inconvenience in practice, and especially so, since any capacity to earth of the ilrst unit, the oscillator, is put in parallel with the feed-back line, thus producing deleterious effects uponthe frequency characteristic of feed-back. There-are known methods of.

applying voltage feed-back whichavoid these disadvantages, but such known methods are complicated'and introduce other compensating disadvantages.

According tc this invention, vcitag'c iced-back is effected between a point in a chain of electron however, will result only in an improved frequency characteristic at the expense of a'slight .loss of gain.'

Preferably provision isv made for varying thel effective feed-back by varying the mutual conductance presented by the cathode follower. Such mutual conductance variation may be effected in any suitable way, for example, by con-v trolling one or more of the electrodes of the cathode follower,` but a preferred way is to employ a condenser shunted cathode resistance and to vary the said resistance. This method of control has the advantage that the'phase changing characteristics ofthe feed-back network are not varied appreciably with variation of the degree of feed-back. This is of particular advantage succeeding multi-stage amplifier arrangement,

the latter being included within the dotted une rectangle. Referring to F1312 the hlghrpotential output' OTI of the oscillator O is'resistance-capacity-coupled by elements RI and Cl to the control grid GI of the first tube VI of thelmulti-stage amplifier which incorporates in the example illustrated three resistance-capacity-coupled amplifier tubes VI, V2, and V3. The cathode KI of the-rst stage tube Vl is connected to ground and to the low potential output terminal T2 of the oscillator through a series circuit comprising a capacity shunted selfbias resistance unit BRI and a cathode resistance CR of low value. Associated with the tube Vl is a cathode follower tube CFV, whose cathode FKis connected through a variable resistance VR shunted by a condenser C2 to the point between the self-bias resistance and the cathode resistance in the cathode leg of the valve VI. Preferably, though not necesssarily, the cathode follower tube is an electron multiplier, but in any event it should have as high a mutual conductance as practicable. Voltage taken` from a' suitable point P in the anode circuit of the last tube V3 of the amplier is fed to the control electrode FG of the cathode follower tube CFV. The cathodes K2, K3 of the second and third tubes V2,

` V3 of the` amplier A are grounded in the usual way, each through its own capacity shunted selfbias resistance unit BRE, BRS. f It will be seen that with this arrangement bot the oscillator and the amplifier units may be VUgrounded on the low potential side, despite the provision of voltage feed-back. Further, the degree of voltage feed-back is\easily and eiciently controllable by varying the value of the resistance VR, which' is common -both to the follower tube CFV and to the rst tube VI of the amplifier.

If desired, instead of employing a single electron discharge device as the cathode follower, a

number oi' devices operated in parallel, may be employed. By operating a number of tubes in parallel in this way, a high effective mutual conductance may be obtained for the cathode follower even if the individual Vtubes constituting` charge tube having a high mutual conductance nected between ground and the cathode of a discharge Ytube in a preceding one of said stages, and a connection from an intermediate point on said cathode resistor to the cathode of the tube in said feed-back circuit.

2. An amplifier system comprising first, second and third electron discharge tube stages, a feed-back circuit arranged to derive output energy from said l(.hird stage for control of the gain in said rst stage, a resistor connected between the cathode -of the first stage and ground, a capacitor shunting a portion of said resistor adjacent the cathode,v said feed-back circuit being connected to the junction between the shunted and unshunted portions of said resistor, means including a discharge tube in said feed-back circuit for amplifying said feed-back energy, said discharge tube having a grid under control of said feed-back energy and a cathode connected to ground through a capacity-shunted" resistor and thence through the unshunted portion of the flrst said resistor, and means for adjusting the degree of feed-back to an optimum value.

3. A system according to claim 2 wherein said discharge tube in the feed-back circuit Vis constituted as an electron multiplier.

a. A system according to claim 2 wherein the last said means isv constituted by a device for varying the impedance of that resistor which is individual to the cathode circuit of said feed back amplifier tube. t,

5. In a three-stage amplifierl system, a feedback circuit arrangement comprising means for A`amplifying -the feed-back energy derived from the output side of the third stage for controlling the gain in the first stage, said means comprising a discharge tube of the velectron multiplier type wherein there is a grid under control of said output energy from the third stage and a cathode connected to an intermediate point on a cathode resistor providedfor the tube in said rst stage, and adjustable means including a variable resistor in the cathode circuit of said electron multiplier tube for varying the degree of feed-back without appreciably disturbing the phase of the feed-back energy applied to said first stage.

6. The combination according to claim 1 in which said discharge tube in the feed-back circuit is constituted as an electron multiplier.

7. The combination according to claim v1 in Vwhich the last mentioned connection is resistive.

OSWOLD EDWARD KEALL. 

